It's difficult to imagine a world without artificial light. But it comes with a price. Over the years, scientists have learned more and more about how light pollution is harming the planet's numerous animals. Here's what we know so far.

If you've ever lived or visited a rural area, you know that the night sky looks vastly different there than it does in a city. Simply put: The number of stars in the sky takes your breath away. The differences in the night sky are due to light pollution — excessive or misdirected artificial light from streetlights, electronic billboards, cars, fishing boats and a host of other sources.

For many years, light pollution was discussed solely in terms of the night sky, as it was known to be a bane to astronomy. In order to really see the stars, galaxies, planets and other objects in space, astronomers were forced to build their observatories and view the night sky far from bright cities.

But numerous studies, particularly in the 1980s and 1990s, began showing how light pollution negatively affects various species. In 2004, scientists with the Urban Wildlands Group, a California-based conservation organization, coined a term for this type of light pollution: Ecological light pollution. "We describe artificial light that alters the natural patterns of light and dark in ecosystems as 'ecological light pollution,'" they write in their study on the matter.

Ecological light pollution, whether from sky glow or direct glare, is now known to affect a broad range of species in different ways. Let's go over some of them.

Sea Turtles

Sea turtles have recently become a kind of "poster child" for the nasty effects of ecological light pollution. Scientists first began documenting how artificial light near beaches affects the reptiles decades ago.

Most sea turtles lay their eggs on beaches at night, when it's dark out. But several studies in the late 70s and early 80s found a strong correlation between bright beaches and a lower number of nesting sites in Florida. Loggerheads and green turtles avoided laying their eggs on these beaches.

It's not that the turtles would come onto the lit beach and decide against laying their eggs; most of them wouldn't even leave the water, while some would would start to leave, and then go back into the ocean. Additionally, the turtles still nested on Florida beaches exposed to lower levels of artificial light, but at much lower numbers. One study even reported that some turtles wouldn't emerge if they even saw the glow of urban light behind the dune.

In 1992, Bair Witherington, a sea turtle researcher with the Florida Fish and Wildlife Research Institute, experimentally showed that artificial light deters nesting activity of sea turtles when he illuminated portions of nesting beaches with different types of light. Interestingly, white light from mercury-vapor lamps discouraged the turtles, whereas yellow light from low-pressure sodium-vapor lamps did not.

In effect, light pollution around nesting beaches represents a type of habitat loss for the turtles, according to Witherington. When these turtles decide not to lay their eggs on their preferred beaches, they turn to less favorable nesting sites, which may not be safe for eggs or hatchlings.

It's well known that light pollution affects migrating birds. During migration, birds use a number of natural compasses to keep on the right path, including the Earth's magnetic field, landmarks and celestial cues (Sun, moon and the stars). Light pollution from tall buildings can disorient migrating birds, especially during poor weather conditions and after midnight, when they tend to fly lower.

Artificial lights pose another threat to migrating birds: Entrapment. There have been numerous documented cases of large groups of birds flying into light beams and becoming trapped, unwilling to leave, wasting precious time and energy needed to complete their migration. In 2010, an estimated 10,000 birds became trapped in the 9/11 Tribute in Light above Ground Zero in New York City.

Ecological light pollution has dire consequences for fledgling seabirds, too. During their first flight out of the nesting colony, fledglings can become disoriented by artificial lights, and either fly into buildings or become exhausted and land on the ground (a phenomenon called "fallout"). Once on the ground, the birds often cannot fly again — they lack the energy and need a long runway to take off.

But attraction and disorientation aren't the only ways that ecological light pollution affects birds — it alters their mating life, too. A 2010 study showed that artificial night lighting causes songbirds to sing their morning songs earlier, increases the number of offspring males sired with females other than their primary mates and causes female blue tits to lay their eggs an average of 1.5 days earlier. Though these changes may not seem all that bad, lead researcher Bart Kempenaers suggests in a release that there's a cost:

"Earlier singing during the morning may come at a cost to males," [Kempenaers] said, noting that they may get less sleep and may be at higher risk of predation. "Second, females are thought to engage in extra-pair copulations with high-quality sires to increase the quality of their offspring. These females may use early singing as a cue reflecting male quality. Light pollution may disrupt the link between the cue — early singing — and male quality, so that females would end up having their offspring sired by lower-quality males. These costs — if they exist — will be hard to measure."

Insects

We've known for a long time that many insects are attracted to light, a fact that's spawned zapping light traps for the pests, as well as the common quip, "Like a moth to a flame…" So, it should come as no surprise that light pollution is changing insect ecosystems.

The findings are still preliminary so it's not clear what impacts — if any — result from these changes in invertebrate communities, but the researchers suggest there may be some cause for concern, considering that invertebrates are crucial players in pollination and decomposition.

How ecological light pollution harms other species is a bit more clear. For example, scientists have long known dry asphalt roads are mayfly traps. When light from streetlights and other sources bounce off asphalt, it becomes horizontally polarized (the light waves are aligned along one plane) — mating and egg-laying mayflies are attracted to this light, thinking the asphalt is water, which they normally breed on. Mayflies are drawn to the roads en masse to breed, but their eggs cannot survive in this environment.

But polarized light doesn't only disturb the reproduction of mayflies. A 2009 study suggests that polarized light pollution is a problem for some 300 aquatic insects, which have been found to lay eggs on various surfaces, including cars, cement floors and glass planes, in addition to asphalt roads. The eggs have no chance of hatching on these surfaces, and when the populations of these insects decrease, it could have consequences for the animals that eat them (birds, especially). Furthermore, if aquatic insects are hanging around roads and cars more often, so are the insect-eating birds.

And, of course, artificial lights are bad for moths, which find the lights irresistible (for reasons that aren't quite clear yet) — rather than mating or searching for food, they spend their time flying around the lights, exhausting themselves. Light pollution may be devastating moth populations, some reports suggest.

But wait, there's more

A number of other species are also affected by ecological light pollution.

For instance, light pollution can affect the vertical migration of zooplankton. The zooplankton Daphina migrates up and down the water column in lakes, in accordance with the level of ambient light — they avoid lit areas so predatory fish don't eat them. In 2000, researchers found that nighttime light pollution limits how high Daphina ascends, which in turn affects how much of the algae-forming phytoplankton they eat. Decreased predation leads to algae blooms that can kill lake life and lower water quality.

Surprisingly, even bats are not immune to light pollution. Though bats navigate their world using echolocation (sound), they are able to sense light. When insects crowd around streetlights and other light sources, fast-flying bats readily gobble them up; but slow-flying bats naturally stay away from light, and thus may be outcompeted by their faster cousins.

Additionally, light pollution alters the flight routes of slow-flying bats, such as the lesser horseshoe bat, causing them to take longer routes that are more energetically costly and may have a higher predation risk from birds of prey.

What can we do?

We've now gone over a lot of different ways that light pollution is altering the ecological landscape and influencing the lives of numerous species. But is there anything we can do about it?

Arguably the simplest approach to managing night-time light pollution is to prevent areas from being lit in the first place, limit the installation of lighting devices or remove lighting devices where these are already in place. When carried out over very large areas, this will prevent or localize the problem of night-time light pollution, at least for those organisms that do not disperse or migrate over longer distances (and hence do not encounter light pollution elsewhere). However, diffuse illumination from artificial skyglow may remain an issue, even tens (and possibly hundreds) of kilometres from urban light sources.

If this isn't possible, the authors suggest limiting how long lights are left on throughout the night, or even using less intense lights. Another option would be to reduce the "trespass of lighting" — that is, install lights that are more focused and don't shine in unwanted or unnecessary areas. And finally, they suggest changing the spectral composition of light pollution (as we've learned with the discussion on turtles, different lights affect species differently).

If you are interested in learning more about these possible paths to reducing ecological light pollution, or hearing about how far people have come with implementing these solutions, check out the review in the Journal of Applied Ecology — it is currently available for everyone.